Generally, a steam turbine installed at nuclear plants and thermal plants is used to convert heat energy of steam to mechanical rotative energy, by colliding high-temperature high-pressure steam with blades arranged in radial directions. When normally operating, the steam turbine performs at about 30 to 60 rotations per second. Also, the steam turbine is structured in such a manner that first to third stages are arranged sequentially from the center.
The blades of the steam turbine are not damaged so frequently during power generation. However, once the blades are damaged, the whole facility may be seriously damaged along with stoppage of the power generation operation. Great expenses are incurred to restore the facility. Also, a long repair time is required.
According to statistics regarding power generation, damage of the blades by crack, corrosion, and parts damage occupies about 30% of accidents of the steam turbine. A centrifugal tension generated by rotation of a rotor of the steam turbine and a bending stress generated by influent steam are among the main stresses causing such damages of the blades. Furthermore, vibration at a nozzle inlet caused by unevenness of the influent steam induces continuous fatigue.
In relation to design of the steam turbine, differently from fatigue cracks generated by resonant vibration of the blades, the cracks occurring on the blades are mostly caused by corrosion. Furthermore, such cracks are generated most frequently at a root of the blade, where a body of the steam turbine and the blade are fixed, rather than at the blade.
As illustrated in FIG. 1, a steam turbine may be structured as follows. Blades 110 each having a steam turbine tenon 140 and fixed on a disc 120 are radially arranged through 360 degrees. The discs 120 are arranged in plural stages symmetrically with respect to an axis of a turbine rotor 100. The disc 120 and the blade 130 are interconnected through a blade root 130.
Conventional methods for performing a nondestructive testing of the blade root 130 of the steam turbine can be classified into a manual testing and an automatic testing both using ultrasonic waves. According to the ultrasonic automatic testing, as illustrated in FIG. 2, the turbine rotor 100 is arranged on a roller 141 and rotated by the roller 141, thereby testing the blade root 130. More specifically, a rail 160 is installed parallel with the axis of the turbine rotor 100, and a testing apparatus 150 tests the respective blade roots 130 by moving along the rail 160 in an axial direction of the turbine rotor 100. Referring to FIG. 2, an ultrasonic probe is mounted to an end of a testing arm 170 to obtain signals corresponding to tested parts.
As illustrated in FIG. 3, the testing can be performed remotely by attaching to the turbine 100 a driver 180 which is magnetically driven.
However, the above conventional testing methods are believed to bear various problems as follows and therefore are believed to require certain improvements.
For example, the testing apparatus 150 including the rail 160 weighs much and occupies a large space due to a large volume. Therefore, in a restricted steam turbine room of the power plant, much manpower and equipment are required to operate the system. Especially, since a prop for preventing shaking of the testing apparatus 150 is a kind of heavy goods weighing about 300 kg, it is difficult to handle the testing apparatus 150.
Furthermore, the testing arm 170 for reaching the inside of the steam turbine from the outside is long. Therefore, the ultrasonic probe mounted at the restless end of the testing arm 170 can hardly contact with a tested object by a proper pressure. Furthermore, transmission of ultrasonic energy to the tested object cannot be achieved effectively, thereby deteriorating reliability of the testing.
Moreover, the roller 141 is necessary to rotate the steam turbine at a predetermined speed. In order to test the steam turbine through 360 degrees, a testing apparatus having access to the steam turbine through 360 degrees is required. However, since such a testing apparatus is unavailable, the steam turbine itself needs to be rotated by 360 degrees.
Still further, in order to disassemble, withdraw and transfer the steam turbine to a testing place, significant manpower and time are required. In addition, there exists an operator's risk in handling such a heavy system.
When transferring to a next tested object after completing the test of one steam turbine, reinstallation of the testing apparatus is necessitated, which takes significant time.